1. Technical Field
The technical field is ceramic capacitors. More particularly, the technical field includes co-fired ceramic capacitors that may be embedded in printed wiring boards.
2. Background Art
Passive circuit components embedded in printed wiring boards formed by fired-on-foil technology are known. Known components are separately fired-on-foil. “Separately fired-on-foil” capacitors are formed by depositing a thick-film dielectric material layer onto a metallic foil substrate and firing under thick-film firing conditions, and subsequently depositing a top electrode material over the thick-film dielectric material layer. U.S. Pat. No. 6,317,023 B1 to Felten discloses such a process.
The thick-film dielectric material should have a high dielectric constant (K) after firing. A high K thick-film dielectric is formed by mixing a high dielectric constant K powder (the “functional phase”) with a glass powder and dispersing the mixture into a thick-film screen-printing vehicle. High K glasses can be wholly or partially crystalline, depending on their composition and the amount of high K crystal they precipitate. These glasses are often termed “glass-ceramics.”
During firing of the thick-film dielectric material, the glass component of the dielectric material softens and flows before the peak firing temperature is reached, coalesces, encapsulates the functional phase, and subsequently crystallizes, forming the glass-ceramic. The glass-ceramic, however, does not re-soften and flow on subsequent firings, and its surface is often difficult to adhere to.
Silver and silver-palladium alloys are preferred metals for forming capacitor electrodes because of their relatively small differences in thermal coefficient of expansion (TCE) from the dielectrics used in fired-on-foil capacitors. Small TCE differences result in low stress in the electrode upon cooling from peak firing temperatures. However, silver and silver-containing alloys may be undesirable in some applications because of the possibility of silver migration. In addition, the relatively low melting points of silver and silver alloys preclude their use at higher firing temperatures.
Copper is a preferred material for forming electrodes, but the large TCE differences between copper and thick-film capacitor dielectrics lead to post-firing stresses in the electrodes. The stresses result in electrode cracking. In addition, because pre-fired glass ceramics do not re-soften and flow on subsequent firings, a copper electrode fired on a pre-fired glass-ceramic surface may not adhere well to the glass-ceramic. The electrode may therefore separate from the dielectric. Both cracking and separation result in high dissipation factors.